Children diagnosed with learning disabilities, a disorder that affects 5% to 10% of the general population, exhibit highly variable perceptual and cognitive profiles. Though many factors can contribute to the diagnosis of a learning problem, evidence indicates that a significant segment of the population with learning disabilities has subtle auditory deficits that contribute to delayed and abnormal reading skills. Since timely identification of children with learning disabilities is critical for successful remediation, there is a need for the development of diagnostics that can quickly and objectively assess children's peripheral and central auditory systems.
Over the last 15 years, Northwestern University's Auditory Neuroscience Research Lab has investigated the prevalence and characteristics of auditory disorders in the learning disabled population by simultaneously exploring neurophysiologic (EEG), perceptual and psychoeducational abilities in learning-disabled individuals. The extensive body of work to come out of the lab indicates that speech-evoked auditory brainstem responses are able to reliably and objectively identify children who may be at risk for learning disabilities. In anticipation of clinical use, we have partnered with Bio-logic Systems Corporation to commercialize this novel neurodiagnostic measure. The BioMAP (Biological Marker of Auditory Processing) will be added to the battery of electrophysiological tests on Bio-logic's Navigator Pro system and will enable clinicians to measure auditory brainstem responses evoked by speech stimuli. It is our hope that the BioMAP will usher in a new standard for the study, evaluation, and rehabilitation of auditory processing disorders. The goals of this article are to briefly address some theoretical considerations for speech-evoked brainstem responses, describe the research that led to the BioMAP, and discuss implications of this research for the clinic.
Origin of the BioMAP
Accurate representation of stimulus timing in the auditory brainstem is a hallmark of normal perception. Recording the brainstem's response to sound has long been established as a valid and reliable means of assessing the integrity of the neural transmission of acoustic stimuli. Transient acoustic events induce a pattern of voltage fluctuations in the brainstem, resulting in a waveform yielding information about brainstem nuclei along the ascending central auditory pathway. Disruptions in this systematic progression on the order of fractions of milliseconds are clinically significant in the diagnosis of hearing loss and brainstem pathology.
To further our understanding of the functional relationship between the acoustic structure of speech and the brainstem's response, a reliable and valid means was developed by which to characterize the neural activity of the brainstem in response to the speech sound /da/ (see Figure 1). Measures of both timing and magnitude are used to describe brainstem neural activity to speech, which is characterized by rapid temporal changes and complex spectral distributions.
Timing measures provide insight into (1) the accuracy with which brainstem nuclei synchronously respond to acoustic stimuli (e.g., peak latency, inter-peak interval, and slope), and (2) the fidelity with which the response mimics either the stimulus or another response (e.g., stimulus-to-response correlations, and inter-response correlations). Magnitude measures provide information about (1) the robustness with which the brainstem nuclei respond to acoustic stimuli and (2) the size of a given spectral component within the response.
Years of research in our lab with the speech sound stimulus indicate that brainstem measures relating to the encoding of linguistic information can serve as a biological marker for auditory function in children with language-based learning problems. The results from our work indicate that learning-disabled children consistently exhibit timing deficits in their speech-evoked auditory brainstem responses. Moreover, results suggest that there are important consequences for abnormal brainstem responses, including abnormal reading and auditory processing abilities as well as abnormal cortical processing of speech sounds. Finally, individuals with abnormal brainstem responses to speech have been shown to benefit from auditory training programs more than learning disabled children with normal brainstem responses.
Taken together, these findings provide critical evidence that speech evoked ABRs can readily identify a sub-population (~30%) of children with learning and auditory processing disorders. Furthermore, these data suggest that the speech-evoked ABR can be a useful diagnostic to probe effects of remediation on the central auditory system, and to address basic research questions regarding the nature of the learning-disabled and normal auditory systems.
We anticipate that the BioMAP will have considerable value in the clinic, not only as a diagnostic to assist in the selection of children who are candidates for auditory-based intervention training, but also as a means of assessing the changes brought about by this training. Just as electrophysiological testing has become an important tool in the assessment of peripheral hearing, the availability of a clinical assessment tool that is valid, objective, and non-invasive will enable clinical professionals to assess more fully the auditory processing abilities of children. Thereby, professionals using this tool may be guided to a more accurate and confident level in the diagnosis and treatment of auditory processing disorders.